The present invention relates to the stabilization of rotating machine stators, particularly those of large electromagnetic rotating machines having a vertical axis of rotation.
Many machines of this type, for example large hydroelectric generators, which have a large diameter compared to their axial dimension, include a stator which is relatively thin in the radial direction. Such a stator is composed basically of a frame and stator laminations extending radially inwardly from, and secured to, the frame.
When such a machine is in operation, an attractive magnetic force acts radially in the air gap between the rotor and the stator, which force acts radially inwardly on the stator. The magnitude of such force is a function, inter alia, of machine diameter, so that particularly large forces exist in the case of large diameter machines of the type described above. The magnitude of such forces, together with the small radial dimension of the stator frame of such a machine, results in the creation of a compressive force, known as a hoop force, which can be sufficient to render the stator structure unstable, in the same manner that a slender column becomes laterally unstable when subjected to a sufficiently high vertical load. When a circular, annular body is subjected to a circumferentially distributed, inwardly radially directed force, the body tends to respond by assuming an elliptical shape.
If a stator of the type here under consideration should become deformed into an elliptical shape, even if such deformation is only very slight, one result will be a change in the circumferential distribution of magnetic attractive forces between the rotor and stator since, at any point around the circumference of the air gap, the magnitude of this force depends on the radial dimension or length, of the air gap. In other words, any elliptical deformation of the stator will result in a redistribution of magnetic forces which tends to maintain or even increase the degree of elliptical deformation.
A conventional approach to preventing deformation of the stator of such machines is to dimension the stator to have a sufficient degree of ring stiffness, i.e. a sufficient resistance to such deformation. In particular, this is achieved by giving the stator frame cross section a sufficiently large radial dimension.
However, when the stator frame is stiffened sufficiently in this manner, this can have adverse effects on the stator core laminations. Notably, such laminations can experience buckling because the larger structural cross section of the stator frame induces a higher compressive reaction on the stator core under various operating conditions, when a temperature differential exists between the frame and the core. Such core lamination buckling is highly undesirable because it results in a looseness of the laminations which can permit them to vibrate or break, as well causing wear of the winding insulation.
Generally, buckling occurs when the induced compressive hoop stress in the lamination structure, or stator core, exceeds a critical value. Basically, the sources of Compressive hoop stress in the stator laminations are: radial magnetic pull; radial interactive forces between the stator core and the stator frame, resulting from free thermal expansion of the stator core and counteracting resistance by the stator frame; frictional drag at the foundation; and frictional drag from any upper bracket structure. In general, the level of radial interactive force increases as the cross section of the stator frame increase, i.e. as the stator diameter increases and as the radial dimension of the frame cross section is increased to provide the desired elliptical stability.
Thus, the problems of buckling and elliptical instability, while of equal importance, have heretofore been considered to require conflicting solutions. In particular, prior art solutions to the problem of elliptical stability, which solutions include increasing the radial dimension of the stator frame or firmly securing the stator frame to a rigid foundation, have adverse influences on the problem of lamination buckling. Prior art solutions to the problem of buckling, which involves allowing radial deflections of the stator frame, inherently complicate the elliptical stability problem.